Synovial fluid is a biological fluid that is found in the synovial cavity of the joints (e.g., knee, hip, shoulder) of the human body between the cartilage and synovium of facing articulating surfaces. Synovial fluid provides nourishment to the cartilage and also serves as a lubricant for the joints. The cells of the cartilage and synovium secrete fluid and the fluid lubricates and reduces friction between the articulating surfaces.
Human synovial fluid is comprised of approximately 85% water. It is derived from the dialysate of blood plasma, which itself is made up of water, dissolved proteins, glucose, clotting factors, mineral ions, hormones, etc. The proteins, albumin and globulins, are present in synovial fluid and are believed to play an important role in the lubrication of the joint area. Other proteins are also found in human synovial fluid, including the glycoproteins such as AGP, A1AT and lubricin.
Another compound that is present in human synovial fluid is hyaluronic acid. Hyaluronic acid is also believed to play a role in lubrication. Human synovial fluid further includes other compounds, such as polysaccharides and phospholipids. The phospholipid, dipalmitoylphosphatidylcholine (DPPC), is also present in human synovial fluid. DPPC is generally regarded as surfactant and is also believed to play a role in the lubrication of the joint.
With the increasing use of artificial orthopedic implants in the treatment of orthopedic injury and disease, the tribology of articulating surfaces in orthopedic implants has become an important area of study. As developers of artificial orthopedic implants attempt to prolong the useful life of orthopedic implants, understanding the amount and degree of wear and friction on the articulating surfaces when such devices are implanted in the body has generated ever-increasing interest among those in the orthopedic field. Accordingly, it is now appreciated that the role of synovial fluid in the tribology of the artificial orthopedic implants must be taken into account.
Testing of implants with human synovial fluid is not practical and may not be desirable for at least several reasons. First, there is simply not enough human synovial fluid available to supply those testing the orthopedic implants. Moreover, the composition of human synovial fluid is very patient specific and, therefore, highly variable. The characteristics of synovial fluid derived from humans may vary, and may be affected by such factors as joint physiology and the state of disease in the joint (if any). Thus, even if human synovial fluid were readily and abundantly available, the testing of orthopedic implants in such fluid would likely lead to variable results and potentially mask the results of any study using such fluid.
Thus, substitutes for human synovial fluid have been developed for studying the tribology of orthopedic implants. These substitutes have been developed to mimic or otherwise simulate the properties and characteristics of human synovial fluid when such fluid is in contact with materials commonly used for orthopedic implants such as, but not limited to, ultrahigh molecular weight polyethylene (UHMWPE) and cobalt-chrome.
Certain early substitutes for synovial fluid were based on water and/or a saline solution, such as the well known Ringers solution. However, these solutions did not adequately simulate or behave like human synovial fluid and often resulted in non-physiological wear rates and patterns. Accordingly, other lubricants believed to more closely approximate the properties of human synovial fluid have been developed.
Bovine serum and solutions based on bovine serum have commonly been used as a substitute for human synovial fluid in tribological testing. The bovine serum can either be normal bovine serum, bovine calf serum and/or fetal bovine serum. The bovine serum is often mixed with water and other components, such as buffers and antibiotics. For example, U.S. Patent Application Publication No. 2007/0160680 discloses an artificial lubricant that includes bovine calf serum. The lubricant further includes additives such as hyaluronic acid and phospholipids of the type naturally found in synovial fluid. In addition, antibiotics, a chelating agent and buffers are also included in the artificial lubricant disclosed therein.
U.S. Patent Application Publication No. 2008/0214417 discloses studies that were conducted with an artificial synovial fluid based on bovine calf serum, newborn calf serum and alpha calf serum. Other studies involving the use of bovine serum as an artificial synovial fluid are reported in the non-patent literature, as are studies of other formulations devoid of bovine serum or the constituents thereof. See Ahlroos T., Saikko, V.; “Wear of Prosthetic Joint Materials in Various Lubricants,” Wear, 211(1): 113-119, 1997.
Whether bovine serum adequately mimics human synovial fluid is not, however, without debate. One question that has been raised is whether current bovine serum based lubricants adequately resemble the human synovial fluid when one considers the components that are believed to contribute to lubrication. For example, the ratio of proteins found in human synovial fluid may not be adequately represented by the synthetic lubricants that are based on bovine serum alone. Moreover, inclusion of bovine serum as a whole introduces additional variables which may not be desirable and may skew the results of any tribological testing.
Thus, there continues to be a need for an artificial lubricant that is more similar to human synovial fluid in its composition, including, but not limited to, protein content and protein ratio, and also in certain of its properties, including (but not limited to) its non-newtonian characteristics. There is also a need for an artificial synovial fluid that remains stable for an extended period of time and does not undergo rapid degradation. In addition, there exists a need for an artificial synovial fluid that is reproducible and eliminates the variables that are introduced by some of the current synthetic synovial fluid lubricants.